S-Turns and zigzagging around LNAV path

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I know that this has been a known issue with PMDG aircraft for quite a while, but I only recently started experiencing this in my sim. I run FSX SP1+2 with AS16. Mostly near cruise altitude, my aircraft can hardly ever maintain stable flight. It 'zigzags' around the LNAV path, first either turning to the left or right and then turning in the other direction to compensate for its earlier deviation.

Now, I've read a lot of forum threads about this issue, and unfortunately, most of them are quite outdated and only deal with Active Sky Evolution/2012 settings. I have, however, tried all the 'solutions' listed in these threads; I've checked and unchecked suppress turbulence, variance, gusts, upper level gusts, wind smoothing, and 'allow changes to FS own weather' in the FSUIPC wind menu. I've changed the TurbulenceRate and TurbulenceDivisor values in FSUIPC.ini, dragging them down to lower levels, and put the "TurbulenceScale" to 0.50000 in fsx.cfg. With AS16, I've dragged my Turbulence effect scale, Updraft, Downdraft and Cloud Turbulence levels all down to 0. I've even tried fixing FSX's registry through Flight1's registry repair tool. Unfortunately, none of these edits have resulted in any change to the aircraft's LNAV performance.

I've noticed similar LNAV performance in my NGX, 777 and MD-11. It is by far the worst with the MD-11, in which it almost feels like the aircraft is Dutch rolling.

In case anyone might be knowing about clashes with other addons, these are the non-aircraft, non-scenery addons I use:

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Wow..., first you claim this is a known issue with PMDG aircraft then cite as reference "outdated ... ASE/2012.." You should not assume this is a common problem. It is not.

I can suggest several items to look at but in the end this is a problem with your configuration. First, ensure your controls are calibrated and do not use FSUIPC to calibrate, use the recommendations in the Introduction regarding FSUIPC. Second you want to ensure that the gust and wind variance settings in AS16 are at a realistic setting. This varies with user and configuration, for comparison I set those to about 20%. Third, make sure the roll trim is zero and finally ensure that you are not using panel states or saved flights from previous PMDG versions. Hope this helps.

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In the MD11 it appears to be version dependent. I can't remember which version has the trouble unfortunately. On my old setup, I never ever had LNAV zigzagging in the MD11. In my new one, I do have zigzags especially when close to the next waypoint.

I used to use the MD-11 version that did not have the "L/H configuration) in the loadout manager. This version did not have problems zigzagging regardless of whether Activesky was enabled or not.

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In the MD11 it appears to be version dependent. I can't remember which version has the trouble unfortunately. On my old setup, I never ever had LNAV zigzagging in the MD11. In my new one, I do have zigzags especially when close to the next waypoint.

I used to use the MD-11 version that did not have the "L/H configuration) in the loadout manager. This version did not have problems zigzagging regardless of whether Activesky was enabled or not.

I've never had S turn problems with PMDG sims, or anyone else's complex sim addons. The only time I've seen anything like it was when I was using FSX real time weather, which has sudden and violent windshifts which, in combination with the way FSX applies wind, messes up LNAV control. The fact that no one seems to be reporting it in forums recently (apart from you), suggests that if it was a problem relating to weather simulation it has been solved by improvements in weather simulation.

AS16 needs no modification or assistance from FSUIPC, and that might even be making things worse for you.

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Seems the problem doesn't lie with any weather engine. I fixed my S-Turn issue following Dan's suggestions and the PMDG 747 intro. All I did was recalibrate my joystick using its own drivers and the aircraft now fly flawlessly. @James: If you use a joystick, recalibrate it from its own drivers that were shipped with the hardware. Also, if you use EZDok, drag down the camera effect processor for weather components to 0. That may also help to an extent.

Thank You, guys!

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Seems the problem doesn't lie with any weather engine. I fixed my S-Turn issue following Dan's suggestions and the PMDG 747 intro. All I did was recalibrate my joystick using its own drivers and the aircraft now fly flawlessly. @James: If you use a joystick, recalibrate it from its own drivers that were shipped with the hardware. Also, if you use EZDok, drag down the camera effect processor for weather components to 0. That may also help to an extent.

Thank You, guys!

Thanks for posting your solution.

And yes - EZDok does do some very strange things before you turn off those effects - that is good advice. I don't like many effects, and it took me a day to figure out why all my planes were going along the runway as if it had huge potholes. Good advice.

Cheers,

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Slight null zone issues in your controls set up will cause this. I can't say for certain but I think that the PMDG code controlling null zone / autothrottle behavior changed in some update. Settings that worked previously may cause the porpoising behavior in both lateral and vertical autopilot modes in the updated version.

Changing the sliders in the PMDG menu may solve this, as will changing your controller null zones in FSUIPC or the sim itself should fix this as well.

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Slight null zone issues in your controls set up will cause this. I can't say for certain but I think that the PMDG code controlling null zone / autothrottle behavior changed in some update. Settings that worked previously may cause the porpoising behavior in both lateral and vertical autopilot modes in the updated version.

Changing the sliders in the PMDG menu may solve this, as will changing your controller null zones in FSUIPC or the sim itself should fix this as well.

I can't follow that logic..., simply because the s-turns are occurring in A/P modes and any interaction by controller would disconnect the A/P. Also, how is the module being controlled aware of any null zone? A null zone is where the input to the module being controlled is unchanged by input to the controller. I suspect the source of the problem in this specific case was the camera/weather. That add-on can create havoc if not set up properly.

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I can't follow that logic..., simply because the s-turns are occurring in A/P modes and any interaction by controller would disconnect the A/P. Also, how is the module being controlled aware of any null zone? A null zone is where the input to the module being controlled is unchanged by input to the controller. I suspect the source of the problem in this specific case was the camera/weather. That add-on can create havoc if not set up properly.

i don't use ezdok.

i am certain it is a controller thing because the problem can be immediately rectified by adjusting the pmdg nullzone, and reintroducing it by reverting the settings.

i don't pretend to know why, but i do know that this fixes the problem.

it may be good to set up a small null zone in your basic FSX or fsuipc controls as well.

perhaps it is not a bug, but it is an unwanted behavior and it is definitely caused by the controller sending tiny inputs while engaging autoflight or while it's already engaged. seeing as there are almost no controllers in the consumer world that don't have some center slop or noise, and considering pmdg even put a null zone tool in the software, i think it's fair to say this is part of the aircraft behavior with consumer hardware, and there are tools provided with the aircraft to stop it.

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